Methods of measuring the intensity of an electromagnetic field formed by electromagnetic waves radiated from an electronic device, which have been practiced as a means to cope with EMI (Electromagnetic Interference), include specified ones described below.
For example, an electronic device on which a measurement is performed, i.e., an object to be measured (a device to be tested), is placed in an open space, and a loop antenna or a dipole antenna is placed at a distance of 3 m to 10 m from the object to be measured to make a measurement. In a case when an antenna is placed at such a sufficiently large distance from an object to be measured, only magnetic field components of a remote radiation electromagnetic field can be measured with the antenna if the antenna is a loop antenna, and only electric field components can be measured with the antenna if the antenna is a dipole antenna. If one of the two components of the remote radiation electromagnetic field is measured, the other component can be computed. Methods of measuring in an electric wave darkroom instead of measuring in an open space have also been specified.
In some case, an electromagnetic wave radiation source is identified on an object to be measured. For example, a case is conceivable in which determination is made as to from which position on a circuit board electromagnetic waves are strongly radiated. In such a case, a measurement different from that described above, i.e., a measurement of the electromagnetic field strength in the vicinity of an object to be measured, is performed. Ordinarily, a small loop antenna is brought close to an object to be measured to measure the magnetic field components of an electromagnetic field. That is, the magnetic field components of an electromagnetic field generated by an object to be measured are detected by utilizing electromotive force induced by electromagnetic coupling. The magnetic field components detected in this way are processed by computation processing to obtain the magnitude and phase of the signal. Conventional measuring devices capable of such magnetic field component measurement are, for example, a vector network analyzer, a vector signal analyzer and a spectrum analyzer. A current-voltage distribution in an object to be measured is obtained by using such a measuring device and performing scanning with a sensor such as a loop antenna. Identification of a radiation source is performed in this way.
The above-described method of using an open space or the like requires a wide installation space and a large amount of equipment investment. In recent years, therefore, an evaluation method using a coaxial transmission path called a 1EM cell has attracted attention as a method of evaluating the intensity of radiated electromagnetic waves. In this evaluation method, an object to be measured is placed between an inner conductor and an outer conductor of a coaxial transmission path, and an evaluation is made through an output from one end of the inner conductor. This method has the advantage of enabling evaluation with a piece of equipment of a comparatively small scale.
On the other hand, identifying an electromagnetic wave radiation source with high accuracy by using the above-mentioned loop antenna requires eliminating the influence of electric field components. A shielded loop antenna provided by forming a shield on a loop antenna is frequently used for this reason. The shielded loop antenna is unsusceptible to the influence of electric field components and is, therefore, capable of measuring only magnetic field components with comparatively high accuracy.
An electromagnetic field intensity measuring method and apparatus capable of easily and accurately measuring each of the electric and magnetic field components of an electromagnetic field formed in space with a small and simple piece of equipment is known. For example, Re-published Japanese Patent Publication No. 02-084311 discloses such a measuring method and apparatus. In the method of measuring the electric field intensity and the magnetic field intensity disclosed in Re-published Japanese Patent Publication No. 02-084311, a conductor is placed in an electromagnetic field and a plurality of output currents output from the conductor in different directions with respect to the electromagnetic field are simultaneously measured. The magnitudes of the output currents and the phase difference between the output currents are thereby measured. An electric field component current produced in the conductor by the electric field and contained in the output currents and a magnetic field component current produced in the conductor by the magnetic field and contained in the output currents are computed on the basis of the magnitudes of and the phase difference between the plurality of output currents measured. The electric field strength and the magnetic field strength of the electromagnetic field are computed on the basis of the computed magnitudes of the electric field component current and the magnetic field component current.
When a conductor is placed in a space in which an electromagnetic field is formed, a current combined from a current produced by the electric field (electric field component current) and a current produced by the magnetic field (magnetic field component current) is ordinarily output. An electric field component current in currents output from particular portions of the conductor is constant even when the conductor is changed in orientation relative to the electromagnetic field. On the other hand, a magnetic field component current in the currents output from the particular portions of the conductor changes in magnitude and direction (phase) when the conductor is changed in orientation relative to the electromagnetic field. It is, therefore, possible to measure the magnitudes of and the phase difference between the plurality of output currents output from the conductor in different directions by simultaneously measuring the output currents. The electric field component current and the magnetic field component current contained in the outputs currents are computed on the basis of the magnitudes of the output currents and the phase difference between the output currents, thus enabling the electromagnetic field intensity to be measured with accuracy at the conductor position.    Patent document 1: Re-published Japanese Patent Publication No. 02-084311
In measurement of a noise signal with a spectrum analyzer, however, a spectrum is displayed by measuring the energy at certain frequencies while scanning frequencies, and the measured values fluctuate and vary if the measured signal fluctuates with time. The measured values also fluctuate and vary in the case of measurement by scanning with a sensor. Thus, there is a possibility of failure to perform an accurate distribution measurement. Further, in such a case, factors for variation with time cannot be measured.
In a case where the measured signal levels are low and vary with time, the difficulty in distinction from noise floor (changing randomly due to thermal noise) is considerably increased, resulting in failure to make an accurate measurement. For example, a portable telephone has such a faint internal noise. Evaluation of the influence of this noise on the receiving sensitivity of the portable telephone requires a different measurement.
Further, in the case of measurement of the influence on the receiving sensitivity of a digital communication device, the influence on the receiving sensitivity of the digital communication system is not sufficiently reflected in the maximum and mean values. Therefore the spectrum measurement in the conventional noise measurement alone is not sufficient for the desired relationship with the receiving sensitivity. That is, factors for the magnitude of the measured signal alone do not sufficiently reflect the influence on the receiving sensitivity and, therefore, the relationship between mapping of the electromagnetic field distribution formed by the measured signal and the receiving sensitivity cannot always be made definite. A relationship cannot be not established in which the degree of interference with digital communication is necessarily high at larger peaks of the maximum value of the spectrum or is necessarily low at valleys in spectrum level. The challenge is to perform a frequency measurement considering this relationship.
The present invention has been achieved in consideration of the above-described problems, and an object of the present invention is to provide an electromagnetic field distribution measuring method and an apparatus thereof capable of measuring an electromagnetic field distribution while considering variation with time in measured values, a computer program and an information recording medium.